1. Field of the Invention
The present invention relates to pipe crawlers, and, more particularly, to pipe crawlers with legs that extend to enable them to crawl in larger diameter pipes.
2. Discussion of Background
There are a number of devices that are designed to move through piping for various purposes such as inspection of welds. These include a few crawlers that move with an "inchworm" type of motion. These pipe crawlers have generally a front set and a rear set of legs separated by body pistons or body air cylinders. Both front and rear sets of legs are made up of air cylinders that extend radially to the wall of the piping. The front set of legs will hold the crawler while the rear set is pulled to the first by a retraction of the body air cylinders; then the rear set will hold the crawler as the front set moves forward when the body air cylinders extend. As this sequence is repeated, the crawler advances through the pipe.
See U.S. Pat. Nos. 4,919,223 issued to Eggers, or 4,581,938 issued to Wentzell as examples of pipe crawlers that move with this inchworm motion. In both of these examples, the leg assemblies have "feet" that extend radially outward to engage the inner wall of the pipe. The feet of these carriers are driven by smaller air cylinders. The stroke length of the cylinders need only be long enough to allow the feet to disengage and engage the pipe wall. However, having a longer stroke length enables a pipe crawler to maneuver in pipes having different diameters.
In U.S. Pat. No. 5,018,451 issued May 28, 1991, filed by the same assignee as the present invention, there is described a pipe crawler moving with the inchworm motion and having two additional air cylinders on each leg assembly for providing additional reach for the four feet of that assembly. The crawler, if sized for a minimum pipe of 12 inches and having a stroke length of two inches for each of the air cylinders that operate the feet, can crawl in a pipe having a maximum inner diameter of 16 inches. With the two additional air cylinders to extend the range of the feet, another four inches of pipe diameter are possible for a maximum diameter of 20 inches.
However, there remains a need for a pipe crawler with feet that can extend even farther and preferably without adding undue complexity in design or operation.